Those comments in the Guardian are mostly complete rubbish. Rules: YOu hold the licence from the country you have a passport for, or you can join the Federation of your country of residence. It will still have a GBR code on, though, if you are British by passport entitlement. Perhaps the Kenyans (renowned as a very upright and honest federation..) would not do that, who knows? Anyway he holds a British passport and that's what counts (look at Cricket and Rugby if you want to find "Brits" that only qualify by residence, or grandparents, or whatever is convenient at the time.
Criteriums do come under UCI via local federation regs, but are relaxed about jerseys simply for good commercial reasons, no-one is worried. The spectators pay to see riders wearing TdF jerseys and they oblige. Simple contractural matter.